Many local area networks were proposed in recent years which allow the shared utilization of a bus or ring network by a large number of attached stations. A common problem is the regulation of the access to such a network if the transmission requests occur irregularly so that a fixed time-division multiplex (TDM) time raster is not desirable.
Two major techniques have evolved for regulating access. One is the carrier sense multiple access technique with collision detection (CSMA/CD) in which a station that wants to transmit first listens and only starts sending if the transmission medium is free. If any collision occurs, transmission is retried according to a given algorithm. Another technique is the token access method in which a token representing the right to transmit is passed from station to station so that only one can attempt to transmit at any time.
Token systems are e.g. known from U.S. Pat. Nos. 4,429,405 and 4,482,999, from an article "A token ring network for local data communications" by R. C. Dixon et al., published in IBM Systems Journal, Vol. 22, No. 1/2 1983, pp. 47-62, and from an article "A local communications network based on interconnected token-access rings: A tutorial" by N. C. Strole, IBM Journal of Research and Development, Vol. 27, No. 5, September 1983, pp. 481-496.
In these prior art systems, the transmission medium is a ring on which the access token as well as the data are passed from station to station in a sequential manner. The mentioned patents and publications also disclose the possibility to transmit synchronous or high-priority data at regular intervals by providing higher priority tokens which are only available to authorized stations.
Though these and many other known systems enable to mix a wide variety of different types of traffic, they have some limitation if bulk transfers of large-volume data blocks are frequently required at high speed. Such transfers may be necessary e.g. for the exchange of whole data bases between computers, or for graphic applications in which the information for rapidly changing displayed images must be sent from one location to another.
The necessity of access control circuitry in the transmission path prevents operation of the network at very high data rates though the medium per se would be able to transfer data at such rates. Furthermore, the transmission of a large-volume data block may require several cycles despite a high-speed transmission rate so that during that time the access would be prevented for stations requiring synchronous transmission at regular intervals.
Some systems are known in which the access information is transmitted separate from the data path, e.g. from an article by D. F. Bantz "Decentralized request resolution mechanism", IBM Technical Disclosure Bulletin, Vol. 20, No. 2, July 1977, pp. 853-855. However, these systems do also not solve the problem of service interruption for synchronous traffic stations by bulk data transfers, and do not allow to transmit data over the separate access line.